The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been selected in September 2019 as the 9th Earth Explorer mission of the European Space Agency. The mission aims to measure the Earth’s Top-Of- Atmosphere (TOA) emission spectrum in the spectral region from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). This will fill the current observational gap from space in the far-infrared region (FIR) from 100 to 667 cm-¹ (i.e. from 15 to 100 μm). FORUM measurements will improve the understanding of the climate system by providing, for the first time with high resolution, the spectral features of the far-infrared emission of the Earth with a focus on the contribution to the radiation budget of the continuum absorption of the water vapour rotational bands, on cirrus cloud properties, and on ice/snow surface emissivity. The FORUM mission requires a payload able to spectrally-resolve the Earth's outgoing longwave radiation across the Far-InfraRed (FIR) spectral range with high absolute radiometric accuracy. Characterisation of the surface, atmospheric and cloud/surface heterogeneity in the observed field-of-view is also required to help interpret the measured spectral radiance. These needs dictate the use of two instruments: a spectrometer and an infrared imager. The concepts of both instruments, thoroughly studied in phase A preparatory activities by two independent industrial consortia, are presented in this paper.
On June 30th, 2005 the REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Application and
Development) Fourier transform spectroradiometer performed the first wide-band spectral characterization of the
top-of-atmosphere emitted radiation in the far-infrared with an uncooled instrument. The nadir emitted radiance
has been measured down to 100 cm-1, thus covering a spectral interval that, until now, was nearly unexplored,
and up to 1400 cm-1, including the well characterized atmospheric window region, in which it is possible to
perform comparison and intercalibration with operative instruments. The measurements were performed at an
altitude of 34 km, from a stratospheric balloon launched in tropical region, near Teresina (Brazil). The acquired
spectra have a spectral resolution of 0.5 cm-1. It should be noted that despite the operating spectral range
extending to the far-infrared region, REFIR-PAD does not require any cooled components, thanks to the use of
pyroelectric detectors and an optical scheme that compensates for the instrument self-emission. This work shows
the results of the analysis of the spectra, focusing on the far infrared portion of the atmospheric emitted radiance.
The retrieval of the vertical profiles of water vapour and temperature during the flight is presented. The vertical
resolution of the retrieval is 2 km in the upper troposphere-lower stratosphere (UTLS) region, and lower at
higher altitudes. The comparison with ECMWF for validation is also shown. Besides the characterization of
temperature and water vapour, from the analysis of the emitted radiance useful information can be gathered
about cloud and aerosol contribution to radiation budget.
The spectrally-resolved characterization of the atmospheric emission has a fundamental role in the study of the
Earth radiation balance, and only a measurement performed in a wide spectral range enables us to separate
the contributions to the radiative balance due to the different altitudes, constituents and physical phases. The
REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Application and Development) Fourier
transform spectroradiometer can perform a characterization of the broadband radiative signature of clouds and
aerosols, with the only limitation of the need of low levels of water vapour like those that are obtainable in high
altitude stations during winter. The capabilities of this kind of measurement have been assessed in a series of
test campaign performed in Tuscany during the winter of 2006/2007 when atmospheric emission spectra have
been acquired in various transparency conditions and the evidence of transparency in the far-infrared region
below 600 cm-1 has been demonstrated. REFIR-PAD operates in the spectral range extending from 100 to 1400
cm-1 with a resolution of 0.5 cm-1, using room-temperature detectors and optics and a compact,
misalignment-compensated
design. The instrument, developed at IFAC-CNR, Florence, has been successfully deployed in
several campaigns, both in the ground based zenith-looking geometry and in the nadir-looking balloon borne
configuration. The operating spectral range of the REFIR-PAD spectroradiometer encompasses great part of the
atmospheric emission spectrum, from the relatively unexplored far-infrared region below 400 cm-1, dominated
by water vapour rotational band, to the atmospheric transparency window, where a number of atmospheric
instruments are already operating and can provide intercomparison data.
The REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Applications and Development) Fourier transform spectroradiometer has successfully performed, at the end of June, 2005, a stratospheric balloon flight from Teresina, Brazil. The instrument has provided 8 hours worth of nadir-looking spectra acquired with a resolution of 0.5 cm-1 in the 100 to 1400 cm-1 spectral range, thus covering both the far-infrared range, containing the radiative signature of the upper tropospheric water vapour, and the better-known mid-infrared range, which provides validation with existing instruments. From the analysis of the calibrated spectra we obtain valuable information on the contribution to the Earth's outgoing long-wave radiation of water, both in the vapour and cloud form, in a region of the atmosphere, the upper-troposphere/lower-stratosphere, in which this contribution has a critical role.
A balloon-borne wide-band Fourier transform spectrometer named REFIR-PAD (Radiation Explorer in the Far InfraRed, Prototype for Applications and Development) has been developed at CNR-IFAC to perform the characterisation of the Earth's outgoing long-wave radiation in the far-infrared region. The spectroscopic characterisation of this region is expected to increase greatly our level of knowledge of the radiative effects of water content in the upper troposphere. The REFIR-PAD instrument provides spectrally-resolved nadir-sounding radiance measurements in the 100-1400 cm-1 range, with a 0.5 cm-1 resolution, covering the most part of the Earth's long-wave emission and including both the far-infrared and the better known middle-infrared region. REFIR-PAD was flown as a piggy-back payload on the CNES IASI-LPMAA stratospheric balloon gondola in June 2005 from Teresina, Brazil. The data collected in this mission, will provide valuable information for the development of a future space mission aimed to the operational monitoring of the upper troposphere water vapour and clouds in order to identify their climate signatures.
Between 25-35% of the Earth's outgoing longwave radiation (OLR) lies in the far-infrared (FIR) spectral region from 0- 500cm-1 where the emission is primarily due to water vapour located in the upper and mid troposphere. The local maximum in the absorption spectrum of ice means that high, cold cirrus clouds have a large effect on intensity of the OLR here. To date, no FIR measurements of the OLR have been made from space, resulting in a major gap in our understanding of the Earth's radiative energy budget. Such measurements will provide vital information about the spatial and temporal variability of the OLR with relation to upper tropospheric humidity and clouds which will better constrain radiation parameterisations in general circulation models. REFIR (the Radiation Explorer in the Far-Infrared) is a polarising interferometer designed to bridge this knowledge gap by measuring the OLR from 100-1100cm-1 at a spectral resolution of 0.5cm-1. This instrument's performance is critically dependent on the properties (transmittance and reflectance) of the wire grid polarisers it uses as beamsplitters. These properties have been measured at Imperial College and incorporated into a mathematical (Jones' matrix) model of the interferometer's performance to produce simulated interferograms and spectra. When coupled to a model of detectors suitable for the FIR spectral region, potential spectral noise characteristics of the calibrated radiance spectra produced by REFIR have been modelled. So far, cryogenically cooled detector systems are far preferable to ambient temperature detectors, although measurements with un-cooled devices with suitable accuracies are possible with longer integration times. The effects of the changing scene beneath the interferometer during the interferogram acquisition time have been analysed.
KEYWORDS: Interferometers, Sensors, Fourier transforms, Signal to noise ratio, Spectroscopy, Calibration, Signal detection, Pyroelectric detectors, Far infrared, Space operations
In the framework of the Radiation Explorer in the Far InfraRed space mission for the characterization in the far infrared of the Earth outgoing emission, a breadboard version of the Fourier transform spectrometer, which is the core instrument of the payload package, has been developed. The Fourier transform spectrometer operates in the spectral range of 100-1100 cm-1 with a resolution of 0.5 cm-1, 6.5 s acquisition time, and signal-to-noise ratio better than 100. It is a compact prototype designed both for laboratory applications and for field campaigns, in particular for operations in high-altitude ground-based sites and on-board of stratospheric balloon platforms.
This paper describes the instrument characterization performed in laboratory conditions and under vacuum. The study has allowed to study the trade-off among all the instrument parameters and to test the new optical design of the interferometer, with particular attention to the photolithographic beam splitters and the room-temperature pyroelectric detectors.
The instrument was operated for the first time in a field campaign in June 2004 from 1247~m altitude ground-based site located in the South of Italy. The results of this test and the comparison with the measurements taken with a BOMEM spectrometer, that partially overlaps the REFIR bandwidth in the higher spectral region, are reported. Info can be found at http://radiation.ifac.cnr.it.
KEYWORDS: Sensors, Interferometers, Signal to noise ratio, Fourier transforms, Pyroelectric detectors, Space operations, Spectral resolution, Far infrared, Spectroscopy, Data acquisition
A spectrometer named Radiation Explorer in the Far InfraRed (REFIR) is being proposed for a future space mission aimed at the spectral measurement in the far infrared of the Earth outgoing emission, with particular attention at the spectral regions that are not covered by any current or planned space mission. The instrument requirements include continuous operation in the spectral range of 100-1100 cm-1 with a resolution of 0.5 cm-1, 6.5 s acquisition time, signal-to-noise ration better than 100 in the ranges 400-600 and 650-800 cm-1 and an accuracy of absolute calibration better than 0.5 K at 280 K at least in the range 400-650 cm-1. To meet these requirements, a spectrometer based on a polarising interferometer with a new optical configuration has been designed. The main characteristics include dual input and output ports, optics of the interferometer with full tilt compensation, and measurement of both planes of polarisation of the source on a single detector. In preparation for a possible space mission, a BreadBoard version (REFIR/BB) of the Fourier transform spectrometer has been built. REFIR/BB will allow us to study the trade-off between all instrument parameters, to test the optical layout and to optimise the data acquisition strategy. In perspective the breadboard could be flown for test flight on aircraft or balloon platforms. This paper describes REFIR/BB characteristics and preliminary experimental results with particular attention to the acquisition strategy and the instrument characterisation. Tests were performed both in air, at ground level, and under vacuum.
An analysis of spectral errors induced by the sampling method used for the interferogram acquisition in Fourier-transform spectroscopy is given. A model is presented that can be used for a rigorous determination of the expected spectral noise in the case of different sampling methods as a function of the quality of the implemented instrumental devices. Using this model numerical evaluations are made in the case of the REFIR (radiation explorer in the far infrared) instrument developed for the measurement of the long wavelength Earth's emission from satellite platforms. The different sampling techniques are considered and evaluated for the selection of the most suitable one for this application. The equal time sampling followed by a numerical filter and a re-sampling has shown the capability of best correcting for sampling errors in the case of this instrument. Furthermore, from the general model an approximate expression is derived that can be used for a preliminary estimate with simple analytical calculations of the spectral noise for different sampling methods. In the specific case of REFIR, a comparison between the numerical modeling and the approximate expression has confirmed that the latter gives a good estimation and can be very useful for the assessment of the relative relevance of the different components of the spectral noise induced by sampling errors.
The REFIR (Radiation explorer in the far infrared) project is a study, funded by European Union, of feasibility of a novel space-borne instrument that will measure the atmospheric spectral radiance of the Earth in the broad spectral range 100-1100 cm-1 from space with sufficient spectral resolution (0.5 cm-1) and signal-to-noise ratio (SNR > 100). The main scientific objectives of the REFIR experiment are the measurement of the outgoing FIR radiation at the top of the atmosphere and the improvement of our knowledge of the principal drivers of this flux, e.g. temperature structure, water vapor, and clouds throughout the troposphere-surface system. The REFIR concept consists of a far infrared Fourier transform spectrometer (FTS) as the core instrument, of an embedded imager operating in an infrared 'window', sharing the same bore-sight as FTS, for scene/cloud signature identification in the FIR, of an add-on imager to provide multi-channel imagery, and of an absolute single-pixel radiometer with a single broad-band channel, used to measure the emitted radiation contextually with the spectral measurements. The integration of all the systems leads to a very compact satellite instrumentation, working at room temperature, with an estimated overall mass of 70 kg and a power consumption of 80 W, including electronics. The overall data rate toward the ground station is foreseen to be of 170 kbps before on-board data compression. This work highlights the main technical results at the end of phase-B0 study. The technical solutions adopted for the instrument are outlined and an accurate analysis of performances is shown.
Great hopes currently lie in new all-optical switching devices based on polymer film waveguides. Here, the design of a novel all-optical waveguide frequency converter is reported, the research activity for which was carried out within the framework of the Italian Project on Telecommunications.
Great hopes currently lie in new all-optical switching devices based on polymer film waveguides. Here, the design of a novel all-optical waveguide Frequency Converter (FC) is reported, the research activity for which was carried out within the framework of the Italian Project on Telecommunications.
Second harmonic (SH) generation in optical waveguides is widely studied for its many applications. In order to obtain efficient non-linear phenomena in waveguides, it is very important that the optico-physical characteristics of the fabricated waveguides are exactly those foreseen in the project. This because in practical applications operation wavelength and temperature can rarely be adjusted to achieve phase-matching conditions. The present work describes a model that, one fixed the operation conditions, finds the optico-physical characteristics of Ti:LiNbO3 planar waveguides necessary to obtain SH generation and relates them to the fabrication parameters. Phase-matching conditions between the two interacting guided modes (fundamental and SH) are achieved by exploiting the material birefringence. The investigation is performed at a fundamental wavelength of 1.064 micrometers and at room temperature on Y-cut, X-propagation waveguides. The waveguides, fabricated according to our design, were then characterized at different wavelengths. Experimental tests of SH generation confirmed the goodness of the model proposed.
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